Diocletian's tetrarchy did not last for long after his retirement. When
Maxentius, who had seized Italy and North Africa, and Constantine, who had
succeeded his father to become emperor in 306, had attempted to seize the
empire, a conflict developed between the two: each of whom were attempting to
rule the Roman Empire. In 312, Constantine defeated Maxentius at Turin and
Verona, then at the battle of Milvian Bridge. Constantine chose to share his
power with Lucinius, who controlled the east. However, in 316, Constantine chose
to attack Lucinius and by 324, Constantine had seized Greece and the Balkans.
One man now ruled the entire Roman Empire once again.

Constantine used his power to promote Christianity, the religion that he
adopted after he saw a vision of a cross while fighting the battle at the
Milvian Bridge. Constantine made Christianity the state religion, and in the
process, confiscated temple treasures and built new churches. Constantine took
interest in the church, by participating on church councils at Arles in 314 and
Nicaea in 325. When he died in 337, he was baptized on his deathbed. Constantine
strengthened the security of the empire (especially along the Danube) and he
reformed the army by separating the frontier units from the mobile field army.
As emperor, he built many structures, including buildings in the first capital,
Trier, and in Rome, where he built baths and built the Basilica Nova, whose
ruins still dominate the forum. He was nominated the famous Arch of Constantine
by the senate, to commemorate his victory over Maxentius. Perhaps Constantine's
most impressive achievements was the transformation of the Greek city,
Byzantium, into a new capital, Constantinople, in 330.